So what if we had a kind of overview when you start the campaign, a section called historical background, then we have a description that shows up in the scenario selection area and during loading and finally there is some dialog/ objective information in game.
I mean, in a single scenario you might have multiple objectives I.E. destroy the tower, build a civic center, train 10 hopilites, and you kind of need to get the player to realize they need to do that.